Method of determining grain size



Patented Feb. 23, 1937 UNlTEl) sT-r METHOD OF DETERMINING GRAIN SIZE OF STEEL William Darke Brown and Charles V. Foerstcr, Jr., Pittsburgh, Pa.

N Drawing. Application May 21, 1934, Serial No. 726,873

4 Claims.

This invention is a method of determining the inherent or austenitic grain size of steel, one of the objects being to make such a determination much more quickly than is possible by means of 5 the customary procedure of carburizing a sample of the steel and then examining it by means of a microscope. It is to be understood that the inherent or austenitic grain size of steel is that grain size which steel naturally acquires when heated to an austenitic state, it being well known that the grain size of steel in its austenitic state varies according to the manner in-which the steel is treated during the time it is molten and that this grain size cannot be materially affected by the subsequent heat treatment or working of the steel after it is solidified. The present in vention is not concerned with the grain size that steel is forced to acquire by controlled heat treatment excepting, of course, those instances where the steel is treated so that its inherent grain size is permitted to develop while the steel is in its austenitic state and Where this grain size is permitted to prevail.

According to this new method, a specimen of the steel whose grain size is to be determined is dissolved in a solvent in which the colloidal or finely dispersed oxide inclusions in the steel are insoluble. In some steels, such as those that contain silicon, heavier inclusions will also be insoluble in such a solvent.

Therefore, a gas is internally generated in the resulting solution. This gas rises to the surface of the solution carrying or displacing all but the colloidal or finely dispersed oxides towards the same. The heavier oxides thus displaced may be silica or silicate inclusions in the steel, While the colloidal or finely dispersed oxides which then remain in the solution are believed to consist of finely dispersed alumina inclusions.

Although the separation of the heavier inclusions in the above manner is believed best, it is also possible to remove them by filtering the solution through a irit.

The middle portion of the solution is now withdrawn, the heavier inclusions being now at the solutions surface, and any remaining gas is removed from this middle portion. The grain size of the steel is then calculated by comparing the turbidity of this middle portion of the solution with various other solutions of similar solvents containing various steels in concentrations equal to that of this solution and whose inherent, grain sizes were previously determined by the ordinary microscopic examination of carburized specimens of the same, or in any other manner. The resulting turbidity oi the solution is obviously a relative matter, and may be compared with suitable preparedstandards, such as by means of an optical wedge that is properly calibrated for the purpose. It has been found that the relative turbidity of the solution containing the steel whose grain size is being determined is an accurate indication of the grain size of this steel.

In actual practice, the steel may be dissolved in perchloric acid to which a few drops of hydrofiouric acid may have been added. The resulting solution is then evaporated to drive off the water present and the residue, when cold, dissolved in water. I I Proper manipulation will result in there being sufficient chlorine in this solution of the residue and water so that thedesired gas may be generated by dropping a little hydrogen peroxide into a test tube and then pouring in the solution. In case there is not sufiicient chlorine this element may be added to the water in which the residue was dissolved.

After a short time the middle portion of the solution in the test tube is removed and heated, while under a slight vacuum, to remove any gas.

This solution is then ready for the comparison v of its turbidity with other solutions or a suitable standard, as previously mentioned.

We claim:

1. A method of determining the inherent grain size of steel, including dissolving at least a portion of the steel in a solvent in which the colloidal or finely dispersed oxides in said steel are insoluble, whereby to produce a solution whose relative turbidity indicates the relative inherent grain size of said steel.

2. A method of determining the inherent grain size of steel, including dissolving at least a portion of the steel in a solvent in which the oxide inclusions in said steel are insoluble, and removing all but the colloidal or finely dispersed oxides from the resulting solution, whereby to produce a solution whose relative turbidity indicates the relative inherent grain size of said steel.

3. A method of determining the inherent grain size of steel, including dissolving at least .a portion of said steel in a solvent in which oxide inclusions in said steel are insoluble, producing a gas in the resulting solution which rises to the latters surface carrying or displacing all but the colloidal or finely dispersed oxides theretowards, and removing at least a part of the portion of said solution now containing only the colloidal or finely dispersed oxides, whereby to produce a solution whose relative turbidityindicates the relative inherent grain size of said steel.

4. A method of determining the inherent grain size of steel, including dissolving a specimen of said steel in a solvent in which oxide inclusions in said steel are insoluble, internally generating a gas in the resulting solution that rises to the latters surface carrying or displac- 0 ing all but the colloidal or finely dispersed oxides 

